GlobalISel is a framework that provides a set of reusable passes and utilities
for instruction selection --- translation from LLVM IR to target-specific
Machine IR (MIR).

GlobalISel is intended to be a replacement for SelectionDAG and FastISel, to
solve three major problems:

Performance --- SelectionDAG introduces a dedicated intermediate
representation, which has a compile-time cost.

GlobalISel directly operates on the post-isel representation used by the
rest of the code generator, MIR.
It does require extensions to that representation to support arbitrary
incoming IR: :ref:`gmir`.

NOTE:
The generic MIR (GMIR) representation still contains references to IR
constructs (such as GlobalValue). Removing those should let us write more
accurate tests, or delete IR after building the initial MIR. However, it is
not part of the GlobalISel effort.

The main addition is support for pre-isel generic machine instructions (e.g.,
G_ADD). Like other target-independent instructions (e.g., COPY or
PHI), these are available on all targets.

TODO:
While we're progressively adding instructions, one kind in particular exposes
interesting problems: compares and how to represent condition codes.
Some targets (x86, ARM) have generic comparisons setting multiple flags,
which are then used by predicated variants.
Others (IR) specify the predicate in the comparison and users just get a single
bit. SelectionDAG uses SETCC/CONDBR vs BR_CC (and similar for select) to
represent this.

The MachineIRBuilder class wraps the MachineInstrBuilder and provides
a convenient way to create these generic instructions.

Generic instructions operate on a new kind of register: "generic" virtual
registers. As opposed to non-generic vregs, they are not assigned a Register
Class. Instead, generic vregs have a :ref:`gmir-llt`, and can be assigned
a :ref:`gmir-regbank`.

MachineRegisterInfo tracks the same information that it does for
non-generic vregs (e.g., use-def chains). Additionally, it also tracks the
:ref:`gmir-llt` of the register, and, instead of the TargetRegisterClass,
its :ref:`gmir-regbank`, if any.

NOTE:
We started with an alternative representation, where MRI tracks a size for
each gvreg, and instructions have lists of types.
That had two flaws: the type and size are redundant, and there was no generic
way of getting a given operand's type (as there was no 1:1 mapping between
instruction types and operands).
We considered putting the type in some variant of MCInstrDesc instead:
See PR26576: [GlobalISel] Generic MachineInstrs
need a type but this increases the memory footprint of the related objects

Additionally, every generic virtual register has a type, represented by an
instance of the LLT class.

Like EVT/MVT/Type, it has no distinction between unsigned and signed
integer types. Furthermore, it also has no distinction between integer and
floating-point types: it mainly conveys absolutely necessary information, such
as size and number of vector lanes:

sN for scalars

pN for pointers

<N x sM> for vectors

unsized for labels, etc..

LLT is intended to replace the usage of EVT in SelectionDAG.

Here are some LLT examples and their EVT and Type equivalents:

LLT

EVT

IR Type

s1

i1

i1

s8

i8

i8

s32

i32

i32

s32

f32

float

s17

i17

i17

s16

N/A

{i8, i8}

s32

N/A

[4 x i8]

p0

iPTR

i8*, i32*, %opaque*

p2

iPTR

i8 addrspace(2)*

<4 x s32>

v4f32

<4 x float>

s64

v1f64

<1 x double>

<3 x s32>

v3i32

<3 x i32>

unsized

Other

label

Rationale: instructions already encode a specific interpretation of types
(e.g., add vs. fadd, or sdiv vs. udiv). Also encoding that
information in the type system requires introducing bitcast with no real
advantage for the selector.

Pointer types are distinguished by address space. This matches IR, as opposed
to SelectionDAG where address space is an attribute on operations.
This representation better supports pointers having different sizes depending
on their addressspace.

NOTE:
Currently, LLT requires at least 2 elements in vectors, but some targets have
the concept of a '1-element vector'. Representing them as their underlying
scalar type is a nice simplification.

TODO:
Currently, non-generic virtual registers, defined by non-pre-isel-generic
instructions, cannot have a type, and thus cannot be used by a pre-isel generic
instruction. Instead, they are given a type using a COPY. We could relax that
and allow types on all vregs: this would reduce the number of MI required when
emitting target-specific MIR early in the pipeline. This should purely be
a compile-time optimization.

Additional passes can then be inserted at higher optimization levels or for
specific targets. For example, to match the current SelectionDAG set of
transformations: MachineCSE and a better MachineCombiner between every pass.

NOTE:
In theory, not all passes are always necessary.
As an additional compile-time optimization, we could skip some of the passes by
setting the relevant MachineFunction properties. For instance, if the
IRTranslator did not encounter any illegal instruction, it would set the
legalized property to avoid running the :ref:`milegalizer`.
Similarly, we considered specializing the IRTranslator per-target to directly
emit target-specific MI.
However, we instead decided to keep the core pipeline simple, and focus on
minimizing the overhead of the passes in the no-op cases.

This pass translates the input LLVM IR Function to a GMIR
MachineFunction.

TODO:
This currently doesn't support the more complex instructions, in particular
those involving control flow (switch, invoke, ...).
For switch in particular, we can initially use the LowerSwitch pass.

The IRTranslator (using the CallLowering target-provided utility) also
implements the ABI's calling convention by lowering calls, returns, and
arguments to the appropriate physical register usage and instruction sequences.

Aggregates are lowered to a single scalar vreg.
This differs from SelectionDAG's multiple vregs via GetValueVTs.

TODO:
As some of the bits are undef (padding), we should consider augmenting the
representation with additional metadata (in effect, caching computeKnownBits
information on vregs).
See PR26161: [GlobalISel] Value to vreg during
IR to MachineInstr translation for aggregate type

The IRTranslator lowers Constant operands into uses of gvregs defined
by G_CONSTANT or G_FCONSTANT instructions.
Currently, these instructions are always emitted in the entry basic block.
In a MachineFunction, each Constant is materialized by a single gvreg.

This is beneficial as it allows us to fold constants into immediate operands
during :ref:`instructionselect`, while still avoiding redundant materializations
for expensive non-foldable constants.
However, this can lead to unnecessary spills and reloads in an -O0 pipeline, as
these vregs can have long live ranges.

TODO:
We're investigating better placement of these instructions, in fast and
optimized modes.

This pass transforms the generic machine instructions such that they are legal.

A legal instruction is defined as:

selectable --- the target will later be able to select it to a
target-specific (non-generic) instruction.

operating on vregs that can be loaded and stored -- if necessary, the
target can select a G_LOAD/G_STORE of each gvreg operand.

As opposed to SelectionDAG, there are no legalization phases. In particular,
'type' and 'operation' legalization are not separate.

Legalization is iterative, and all state is contained in GMIR. To maintain the
validity of the intermediate code, instructions are introduced:

G_MERGE_VALUES --- concatenate multiple registers of the same
size into a single wider register.

G_UNMERGE_VALUES --- extract multiple registers of the same size
from a single wider register.

G_EXTRACT --- extract a simple register (as contiguous sequences of bits)
from a single wider register.

As they are expected to be temporary byproducts of the legalization process,
they are combined at the end of the :ref:`milegalizer` pass.
If any remain, they are expected to always be selectable, using loads and stores
if necessary.

Currently the API is broadly similar to SelectionDAG/TargetLowering, but
extended in two ways:

The set of available actions is wider, avoiding the currently very
overloaded Expand (which can cover everything from libcalls to
scalarization depending on the node's opcode).

Since there's no separate type legalization, independently varying
types on an instruction can have independent actions. For example a
G_ICMP has 2 independent types: the result and the inputs; we need
to be able to say that comparing 2 s32s is OK, but the s1 result
must be dealt with in another way.

As such, the primary key when deciding what to do is the InstrAspect,
essentially a tuple consisting of (Opcode, TypeIdx, Type) and mapping to a
suggested course of action.

An example use might be:

// The CPU can't deal with an s1 result, do something about it.
setAction({G_ICMP,0,s1},WidenScalar);// An s32 input (the second type) is fine though.
setAction({G_ICMP,1,s32},Legal);

TODO:
An alternative worth investigating is to generalize the API to represent
actions using std::function that implements the action, instead of explicit
enum tokens (Legal, WidenScalar, ...).

TODO:
Moreover, we could use TableGen to initially infer legality of operation from
existing patterns (as any pattern we can select is by definition legal).
Expanding that to describe legalization actions is a much larger but
potentially useful project.

TODO:
Types which have a size that isn't a power of 2 aren't currently supported.
The setAction API will probably require changes to support them.
Even notionally explicitly specified operations only make suggestions
like "Widen" or "Narrow". The eventual type is still unspecified and a
search is performed by repeated doubling/halving of the type's
size.
This is incorrect for types that aren't a power of 2. It's reasonable to
expect we could construct an efficient set of side-tables for more general
lookups though, encoding a map from the integers (i.e. the size of the current
type) to types (the legal size).

Vectors first get their element type legalized: <A x sB> becomes
<A x sC> such that at least one operation is legal with sC.

This is currently specified by the function setScalarInVectorAction, called
for example as:

setScalarInVectorAction(G_ICMP, s1, WidenScalar);

Next the number of elements is chosen so that the entire operation is
legal. This aspect is not controllable at the moment, but probably
should be (you could imagine disagreements on whether a <2 x s8>
operation should be scalarized or extended to <8 x s8>).

It iteratively maps instructions to a set of per-operand bank assignment.
The possible mappings are determined by the target-provided
:ref:`RegisterBankInfo <api-registerbankinfo>`.
The mapping is then applied, possibly introducing COPY instructions if
necessary.

It traverses the MachineFunction top down so that all operands are already
mapped when analyzing an instruction.

This pass could also remap target-specific instructions when beneficial.
In the future, this could replace the ExeDepsFix pass, as we can directly
select the best variant for an instruction that's available on multiple banks.

Cross-Bank Copies: copyCost --- the cost of a COPY from one bank
to another.

Default Mapping: getInstrMapping --- the default bank assignments for
a given instruction.

Alternative Mapping: getInstrAlternativeMapping --- the other
possible bank assignments for a given instruction.

TODO:
All this information should eventually be static and generated by TableGen,
mostly using existing information augmented by bank descriptions.

TODO:
getInstrMapping is currently separate from getInstrAlternativeMapping
because the latter is more expensive: as we move to static mapping info,
both methods should be free, and we should merge them.

NOTE:
On AArch64, we are considering using the Greedy mode even at -O0 (or perhaps at
backend -O1): because :ref:`gmir-llt` doesn't distinguish floating point from
integer scalars, the default assignment for loads and stores is the integer
bank, introducing cross-bank copies on most floating point operations.

The instance is provided by the subtarget, so that it can specialize the
selector by subtarget feature (with, e.g., a vector selector overriding parts
of a general-purpose common selector).
We might also want to parameterize it by MachineFunction, to enable selector
variants based on function attributes like optsize.

The simple API consists of:

virtualboolselect(MachineInstr&MI)

This target-provided method is responsible for mutating (or replacing) a
possibly-generic MI into a fully target-specific equivalent.
It is also responsible for doing the necessary constraining of gvregs into the
appropriate register classes as well as passing through COPY instructions to
the register allocator.

The InstructionSelector can fold other instructions into the selected MI,
by walking the use-def chain of the vreg operands.
As GlobalISel is Global, this folding can occur across basic blocks.

TableGen will import SelectionDAG rules and provide the following function to
execute them:

boolselectImpl(MachineInstr&MI)

The --stats option can be used to determine what proportion of rules were
successfully imported. The easiest way to use this is to copy the
-gen-globalisel tablegen command from ninja -v and modify it.

Similarly, the --warn-on-skipped-patterns option can be used to obtain the
reasons that rules weren't imported. This can be used to focus on the most
important rejection reasons.

PatLeafs cannot be imported because their C++ is implemented in terms of
SDNode objects. PatLeafs that handle immediate predicates should be
replaced by ImmLeaf, IntImmLeaf, or FPImmLeaf as appropriate.

There's no standard answer for other PatLeafs. Some standard predicates have
been baked into TableGen but this should not generally be done.

Custom SDNodes should be mapped to Target Pseudos using GINodeEquiv. This
will cause the instruction selector to import them but you will also need to
ensure the target pseudo is introduced to the MIR before the instruction
selector. Any preceeding pass is suitable but the legalizer will be a
particularly common choice.

ComplexPatterns cannot be imported because their C++ is implemented in terms of
SDNode objects. GlobalISel versions should be defined with
GIComplexOperandMatcher and mapped to ComplexPattern with
GIComplexPatternEquiv.

The following predicates are useful for porting ComplexPattern:

isBaseWithConstantOffset() - Check for base+offset structures

isOperandImmEqual() - Check for a particular constant

isObviouslySafeToFold() - Check for reasons an instruction can't be sunk and folded into another.

There are some important points for the C++ implementation:

Don't modify MIR in the predicate

Renderer lambdas should capture by value to avoid use-after-free. They will be used after the predicate returns.

Only create instructions in a renderer lambda. GlobalISel won't clean up things you create but don't use.

The current "stage" in the :ref:`pipeline` is represented by a set of
MachineFunctionProperties:

legalized

regBankSelected

selected

MachineVerifier

The pass approach lets us use the MachineVerifier to enforce invariants.
For instance, a regBankSelected function may not have gvregs without
a bank.

TODO:
The MachineVerifier being monolithic, some of the checks we want to do
can't be integrated to it: GlobalISel is a separate library, so we can't
directly reference it from CodeGen. For instance, legality checks are
currently done in RegBankSelect/InstructionSelect proper. We could #ifdef out
the checks, or we could add some sort of verifier API.

The initial goal is to replace FastISel on AArch64. The next step will be to
replace SelectionDAG as the optimized ISel.

NOTE:
While we iterate on GlobalISel, we strive to avoid affecting the performance of
SelectionDAG, FastISel, or the other MIR passes. For instance, the types of
:ref:`gmir-gvregs` are stored in a separate table in MachineRegisterInfo,
that is destroyed after :ref:`instructionselect`.

FastISel Replacement

For the initial FastISel replacement, we intend to fallback to SelectionDAG on
selection failures.

Currently, compile-time of the fast pipeline is within 1.5x of FastISel.
We're optimistic we can get to within 1.1/1.2x, but beating FastISel will be
challenging given the multi-pass approach.
Still, supporting all IR (via a complete legalizer) and avoiding the fallback
to SelectionDAG in the worst case should enable better amortized performance
than SelectionDAG+FastISel.

NOTE:
We considered never having a fallback to SelectionDAG, instead deciding early
whether a given function is supported by GlobalISel or not. The decision would
be based on :ref:`milegalizer` queries.
We abandoned that for two reasons:
a) on IR inputs, we'd need to basically simulate the :ref:`irtranslator`;
b) to be robust against unforeseen failures and to enable iterative
improvements.

Support For Other Targets

In parallel, we're investigating adding support for other - ideally quite
different - targets. For instance, there is some initial AMDGPU support.